Typically, a base station communicates with a mobile station using a one-dimensional antenna arranged in a horizontal direction. In recent years, in a macro-cell and a micro-cell, the number of radio transmission units sharply increases and the number of mobile stations also increases. To improve signal transmission quality between a base station and a mobile station and to increase throughput of a radio communication system, a full-dimensional beam forming technology has been proposed. According to a full-dimensional beam forming technology, a base station communicates with a mobile station using a two-dimensional antenna array arranged in a horizontal direction and a vertical direction. In the two-dimensional antenna array, antennas are arranged not only in the horizontal direction for horizontal beam control but also in the vertical direction (height direction) for vertical beam control. Thus, full-dimensional beam forming can be realized. Through the full-dimensional beam forming, a higher beam forming gain can be obtained and higher signal transmission quality can be also achieved for mobile stations located at different positions.
It is necessary for a base station to transmit reference signals (RSs) to a mobile station in order for the mobile station to estimate channel state information (CSI) before the base station transmits downlink data to the mobile station. Specifically, a base station can transmit beam-formed (BF) CSI-RS (hereinafter referred to as “BF CSI-RS”) beams or non-precoded (NP) CSI-RS (hereinafter referred to as “NP CSI-RS”) beams to a mobile station. The mobile station can estimate a channel based on the received beams and feeds a channel estimation result back to the base station. Thus, the base station can transmit downlink data based on the channel estimation result. However, the BF CSI-RS beams typically involve a dedicated transmission and processing method and the NP CSI-RS beams also typically involve a dedicated transmission and processing method. In other words, compatible transmission and processing may not be applied to reference signals in different communication modes. Accordingly, a communication technology for enhancing a capability to process reference signals is preferable.